Product made by method of entraining dislocations and other crystalline defects

ABSTRACT

A substrate, such as a film of thermally grown silicon dioxide on a silicon wafer is coated with a thin film of polycrystalline or amorphous silicon in the thickness range 0.05-10μ deposited by chemical vapor deposition. An encapsulation layer that is a composite of 2 μm thickness SiO 2 , 30 nm of Si 3  N 4  is deposited on the thin film. A pattern of stripes is created on this encapsulation layer made of materials, such as titanium, silicon, silicon dioxide and photoresist. A long and narrow molten zone is created in the film with its long axis oriented perpendicular to the lines and is moved with a movable strip-heater over in a direction parallel to the lines in the recrystallization process to establish dislocation and crystalline defects in the film entrained to follow the pattern of the stripes at locations related to the stripes.

The Government has rights in this invention pursuant to Contract No.N00014-79-C-0908 awarded by the U.S. Dept. of the Navy.

This application is a division of application Ser. No. 391,130, filed6/23/82, now U.S. Pat. No. 4,479,846.

This invention relates in general to improving the quality of thincrystalline films by confining, or entaining, crystalline boundaries topredetermined locations.

Thin films of crystalline material are important in many fields ofscience and technology. In semiconductor electronics there isconsiderable interest in obtaining high-quality semiconductor films oninsulating substrates, especially silicon (Si) on silicon dioxide(SiO₂). In recent years numerous investigators have produced large-grainpolycrystalline films of Si on SiO₂ by melting fine-grainpolycrystalline Si using a laser or strip-heater and then allowing themelt to resolidify. Such processes are generally referred to as laserrecrystallization, strip-heater recrystallization or zone-meltingrecrystallization. In some cases, the molten material is given directaccess to a single-crystal silicon substrate through an opening in theSiO₂. The single crystal Si substrate then serves as a seed, enablingthe Si film which grows laterally across the SiO₂ to have the samecrystallographic orientation as the seed. However, low-angle grainboundaries or subboundaries generally appear in the "single orientation"recrystallized Si films. Subboundaries are also present within grains offilms produced without substrate seeding. Both large-angle and low-anglegrain boundaries are undesirable for microelectronic devices, especiallyhighly integrated circuits. (Henceforth both types of boundaries will becalled simply "boundaries" or "crystalline boundaries".) The eliminationor confinement of all such boundaries is highly desirable. It istherefore an object of this invention to provide a technique which, whenused in conjunction with recrystallization of thin films, confinesboundaries to predetermined positions.

It is a further object of this invention to provide a technique whichwill induce undesirable impurities to concentrate at these samepositions.

It is a further object of this invention to achieve one or more of thepreceding objects using patterns produced by planar fabricationprocesses to define the predetermined positions.

According to the invention, a film to be processed is located on top ofa substrate and, in some cases, is covered with an encapsulation layer.A pattern, generally having the form of uniformly-spaced parallelfeatures, is intentionally created in or on the substrate, or in or onthe encapsulation layer, or in the film. The film is processed bypassing a molten zone through it in such a way that dislocations andother crystalline defects which occur in the film are entrained tofollow the pattern. In this way these dislocations and the crystallinedefects are located at predetermined positions.

Numerous other features, objects and advantages of the invention willbecome apparent from the following specification when read in connectionwith the accompanying drawing in which:

FIG. 1 is a cross-sectional view of a sample with a pattern forproducing entrainment of crystalline boundaries;

FIG. 2 is an enlarged top view of a portion of the sample of FIG. 1illustrating the parallel relationship between the axis of theentrainment pattern and the direction of motion of the molten zone;

FIG. 3 is a perspective view of a movable strip-heater oven used forzone-melting recrystallization; and

FIGS. 4A and 4B are micrographs illustrating recrystallization by thezone-melting process without and with an entrainment pattern,respectively.

With reference now to the drawings, there is shown in FIG. 1 across-sectional view of a sample on which there has been created apattern designed to produce entrainment of crystalline boundaries, aswell as the localization of various other defects and impurities. Thesubstrate 1 is coated with a film 2 to be recrystallized. A suitablesubstrate is a film of thermally grown SiO₂ on a Si wafer. On top ofthis substrate a thin film 2 to be recrystallized and entrained isdeposited. Experimental work has been mostly with films ofpolycrystalline or amorphous silicon in the thickness range 0.05-10 μm,deposited by chemical vapor deposition (CVD) means. On top of this filmis an encapsulation layer 3. In experimental work this encapsulationlayer has mostly been a composite of 2 μm thickness SiO₂, 30 nm of Si₃N₄. On top of the encapsulation layer, a pattern of stripes 4 iscreated, as shown in cross section in FIG. 1 and in a magnified top viewin FIG. 2. This pattern of stripes is known as the entrainment patternand has been made of a variety of materials, including titanium, amaterial that reflects radiation, silicon, silicon dioxide, andphotoresist, a material that absorbs radiation. The latter was heated toa high temperature to cause carbonization. FIG. 2 illustrates that theaxis of the entrainment pattern is aligned parallel to the direction ofmotion of the molten zone, indicated by the arrow in FIG. 2. Thecomposition, cross-sectional profile, thickness, linewidth and spacing(i.e., spatial period) of the entrainment pattern are subject to controland can be varied over a wide range. A typical spatial period is 100 μm.A pattern more complex than a simple linear grating for example, apattern whose spatial period varies periodically, is also includedwithin the scope of this invention.

Once the sample has been prepared, as indicated in FIGS. 1 and 2, therecrystallization process is carried out by creating a molten zone inthe film 2 and moving this molten zone in a direction parallel to theaxis of the entrainment pattern. Typically, the molten zone is long andnarrow, with its long axis oriented perpendicular to the lines of theentrainment pattern. FIG. 3 shows two heaters, a lower fixedstrip-heater 11 and an upper movable strip-heater 12.

When the zone-melting recrystallization is done without the entrainmentpattern for polycrystalline silicon films over SiO₂, encapsulated with 2μm SiO₂ /30 mm Si₃ N₄, a film is obtained which consists of largecrystalline grains approximately 1 mm wide by several mm long, dependingupon the distance over which the molten zone is scanned (see, forexample, M. W. Geis, H. I. Smith, B-Y. Tsaur, John C. C. Fan, E. W. Mabyand D. A. Antoniadis, entitled "Zone-melting recrystallization ofencapsulated silicon films on SiO₂ -morphology and crystallography",Applied Physics Letters, Vol. 40, p. 158-160, 1982). Within individualgrains there are subboundaries which include arrays of dislocations andother types of defects. The subboundaries form a characteristic patternbut otherwise have random locations, as described in several publishedarticles, including the above cited article in Applied Physics Letters.The major grains boundaries also have random lateral locations. Withinthese films there is a strong preferance for (100) texture to dominate.That is, the (100) planes of grains are predominantly parallel to thesubstrate plane. In addition, as the molten zone is moved along thelength of a Si film, there is a tendency for the crystalline Si whichfreezes at the trailing edge of the molten zone to be dominated bygrains having <100> directions within several degrees of the scandirection.

When an entrainment pattern is used in conjunction with the zone meltingrecrystallization process, described above, a dramatic change in thepattern of crystalline boundaries can be effected: these boundaries canbe made to align parallel to, and with the same spacing as, theentrainment pattern. This entrainment is illustrated graphically inFIGS. 4A and 4B which show a comparison of a film which had beenrecrystallized by the zone-melting process without an entrainmentpattern (FIG. 4A) and the result obtained with an entrainment pattern(FIG. 4B). This result represents a novel use of an artificial planarpattern to control the defect structure of a crystalline thin film in apredetermined way.

In addition to controlling the location of crystalline boundaries,impurities also tend to concentrate at the entrained boundaries.

It is believed that in zone-melting recrystallization of encapsulated Sithe interface between liquid and solid is faceted, and the subboundariesform at the interior corners of this faceted interface as it movesalong. Impurities rejected into the liquid during solidification wouldlikewise tend to concentrate at the interior corners. Thus, it appearsthat to entrain boundaries one needs to control the locations of theinterior corners. In the entrainment method described above, with apattern of carbonized stripes on top of the encapsulation layer, theboundaries, and hence the interior corners, are entrained to lie underthe carbonized stripes. It is believed that the mechanism is thermal;that is, the entrainment pattern modulates the lateral temperatureprofile in the film in a controlled, periodic manner, and the interiorcorners become locked in or "entrained" with this pattern-inducedtemperature profile.

The entrainment of the interior corners of a solidification front neednot depend on temperature modulation alone. For example, it has beendiscovered that under certain circumstances relief structures in asubstrate can entrain crystalline boundaries in a film recrystallizedover that substrate. It is believed that in this case the reliefstructure causes a local indentation or perturbation in thesolidification front. There are numerous versions of artificiallycreated patterns that could be employed to entrain boundaries. Theprinciple of this invention, however, is common to all: produce apattern which alters the contour of the solidification front such thatthe sources of the crystalline boundaries lock in step with theartificial pattern. Another approach would be to create the pattern inthe film itself.

Another approach to entraining crystalline boundaries is to use apattern of stripes of a material whose interfacial tension with thesolidifying material causes a modulation of the position of thesolidification front. This pattern could be put on the substrate or inor on the encapsulation layer 3.

The technique of using an artificial pattern to entrain crystallineboundaries during the recrystallization of a thin film is applicable tomaterials other than silicon. Other important semiconductors, such asGaAs, InP, InSb, GaAlAs, could probably also be recrystallized andentrained in a similar manner. In fact, during the 1960's severalworkers demonstrated the recrystallization of InSb by a process similarto that described above for Si. They did not, however, describe oranticipate the entrainment of crystalline boundaries through the use ofplanar patterns.

There has been described novel apparatus and techniques for entrainingcrystalline boundaries to produce novel articles of manufacture. It isevident that those skilled in the art may now make numerous uses andmodifications of and departures from the specific embodiments describedherein without departing from the inventive concepts. Consequently, theinvention is to be construed as embracing each and every novel featureand novel combination of features present in or possessed by theapparatus and techniques herein disclosed and limited solely by thespirit and scope of the appended claims.

What is claimed is:
 1. A product made by the method of entrainingdislocations and other crystalline defects in a film on a substratewhich method includes the steps of intentionally creating apredetermined pattern in a region embracing at least one of said filmand said substrate to form a patterned region,and processing said filmby passing a heated zone through said film in a predetermined directionto establish dislocations and other crystalline defects in said filmentrained to follow said pattern and located at predetermined positionsrelated to said pattern.
 2. A product in accordance with claim 1 whereincrystalline boundaries form said patterned region.
 3. A product inaccordance with claim 1 wherein the substrate has impurity atoms, theconcentration of the impurity atoms being a function of their positionin said substrate.